In the field of integrated circuits, it is becoming popular to use a technique to transmit electrical signals to a module isolated (insulated) by an insulating device, using an electromagnetic field generated by the insulating device. This technique enables communication between insulated modules by generating electrical signals on the other insulated module (communication partner) through a magnetic field coupling by a transformer or an electric field coupling by a capacitor element.
A signal processing device using such an electromagnetic field typically uses a high-frequency electromagnetic field due to the fact that insulation is performed with a transformer or capacitor element or due to various reasons (e.g. sizes). Hence, a communication scheme is used in which modulation is performed using the high frequency. Frequency shift keying (FSK) communication is known as one of this type of communication schemes. In FSK communication, modulation is performed in such a manner that frequency of the high frequency is switched depending on the value of communication data, and the communication data is demodulated according to the high frequency.
To precisely demodulate the communication data through a reception circuit receiving the communication data, it is necessary to precisely judge a difference in the frequency. The frequency generally depends on the oscillation frequency of the oscillator in a transmission circuit, but the oscillation frequency fluctuates with temperature, power supply voltage, and the like and then deviates from a predetermined value. Besides, demodulation in the reception circuit is also susceptible to temperature, power supply voltage, and the like. Therefore, even with calibration between the transmission circuit and the reception circuit, it is difficult to keep stable demodulation.